Chemical Raman enhancement of organic adsorbates on metal surfaces.

Using first-principles theory and experiments, chemical contributions to surface-enhanced Raman spectroscopy for a well-studied organic molecule, benzene thiol, chemisorbed on planar Au(111) surfaces are explained and quantified. Density functional theory calculations of the static Raman tensor demonstrate a strong mode-dependent modification of benzene thiol Raman spectra by Au substrates. Raman active modes with the largest enhancements result from stronger contributions from Au to their electron-vibron coupling, as quantified through a deformation potential. A straightforward and general analysis is introduced to extract chemical enhancement from experiments for specific vibrational modes; measured values are in excellent agreement with our calculations.

First-principles study of confinement effects on the Raman spectra of Si nanocrystals.

The Raman spectra of Si nanocrystals are studied as a function of nanocrystal diameter using pseudopotential density functional theory and the Placzek approximation. Our calculations reproduce the redshift and broadening of the optical Raman peak with decreasing nanocrystal size, and calculated peak frequencies show good agreement with experimental values. We also find that a surface induced softening of vibrational modes is largely responsible for the Raman redshift, with relaxation of momentum conservation playing only a minor role.